1. Field of Invention
The invention relates to an optical network and, in particular, to a device and method that can determine the conditions of individual optical branches in optical networks.
2. Related Art
Currently, the passive optical network (PON) undoubtedly is the hottest technique for the optical access network. However, the optical fiber monitoring technology is still limited. The current solution of using an optical time-domain reflectometry (OTDR) results in the problem that individual branches are hard to distinguish. In particular, the discrimination of branches of similar lengths is restricted by the OTDR dead zone. This is because that the light pulse for detection can enter each branch of the optical fiber. Likewise, the optical waves reflected from each optical path are received by the OTDR. Therefore, the reflected light in different branches but with the same distance from the OTDR will contribute to the optical receiver of OTDR, thus accumulating on the OTDR trace. In this case, when one optical path has an event occurring, especially when two paths are close in length, then the conventional monitoring technology cannot determine in which branch the event is occurring.
As shown in FIG. 1, a 1:2 splitter 10 is used at a distance of 3.07 km from the front end in an optical network structure with multiple branches, similar to the passive optical network. A 1:8 splitter 20 is further used at a distance of 2.08 km from the previous splitter. The output terminals a through d are coupled to the splitters at 0.42 km, 1.54 km, 1.67 km, and 2.45 km. The output terminals e to h are not connected. The front end of the network employs a conventional OTDR to measure the optical fiber condition of the network, as shown in FIG. 2, showing the path diagram of the OTDR output. Path 2 shows the attenuation of the 1:2 splitter 10. Path 4 shows the attenuation of the 1:8 splitter 20. The output terminal a to the tail of d also show the paths 5, 6, 7, and 8 in the OTDR trace. All the branch conditions are accumulated onto the same path diagram. Although the conditions on the optical network can be displayed, if one of the branches has a problem it can be discovered by comparing with the original trace with no problem conventionally. This is because at the problem point, the trace after the position is changed. The change stops at the end of the branch with a problem. This is the conventional optical fiber condition monitoring method, as disclosed in U.S. Pat. No. 5,187,362.
However, on these two optical paths of similar lengths, such as paths 6 and 7, it is difficult to distinguish them due to the limitation of the OTDR dead zone. If the length of the optical path increases, an OTDR signal with a longer pulse has to be used to detect. In this case, the dead zone also increases so that one is unable to distinguish the 1.54 km and 1.67 km paths. When an even happens in one of them, it is impossible for find what in which path it is using the conventional monitoring method. In addition, the reflected light from all paths overlap and present on the same OTDR trace if the conventional monitoring method is used. Therefore, one cannot detect the optical fiber properties (e.g., transmission attenuation) of individual paths. It is thus imperative to develop a new monitoring technique to solve the discrimination problem.